1. Field
The present invention relates to an antireflection film, a polarizing plate, an image display device and a manufacturing method for antireflection film.
2. Description of Related Art
On an image display device such as a cathode ray tube (CRT) display device, a plasma display panel (PDP), an electroluminescence display (ELD), a vacuum fluorescent display (VFD), a field emission display (FED) and a liquid crystal display (LCD), an antireflection film is provided in some cases for preventing deterioration of contrast due to reflection of natural light at a surface thereof or of glare.
An antireflection film is required to a low reflection index because the antireflection film is provided at a surface of a display for preventing reflection of natural light or occurrence of glare. The antireflection film is also required to a high mechanical strength such as scratch resistance, transparency, or the like. Thus, the antireflection film is formed by layering a substrate, a functional layer such as a hardcoat layer or a high refractive index layer on the substrate, and a low refractive index layer having a suitable thickness and a lower refractive index than one of the substrate, in this order.
As an antireflection film, an antireflection film having a fine unevenness shape on a surface of the substrate, of which a period is shorter than wavelength of visible rays, has been known. Such a fine unevenness structure is a so-called moth-eye structure. Due to the moth-eye structure, a pseudo refractive index incline layer in which a refractive index is continuously varied from air toward a bulk material at an inside of the substrate is formed. That is, by not having an intersurface of refractive index, it is possible to prevent reflection of light.
As an antireflective index layer having a moth-eye structure, JP-A-2011-70116 discloses a manufacturing method for an antireflection film having a moth-eye structure being excellent in adhesion property with a substrate, the manufacturing method including directly applying an antireflection composition containing a binder in which a molecular weight and the number of functional groups are defined on a light transmitting substrate, forming an unevenness structure on a surface of the antireflection film by using a mold for forming an antireflection film, of which a period is shorter than wavelength of visible rays, peeling the mold for forming antireflection film.
JP-B-5,038,748 discloses a manufacturing method for an antiglare film having an unevenness surface, including using two kinds of polymers having specific structures without using a mold to form a phase separation structure by spinodal decomposition from a liquid phase, and continuously curing the precursor.
JP-A-2013-693 discloses a method for forming a fine unevenness structure by radiating far infrared radiation to an antireflection composition as a method for forming a moth-eye structure of which a period is shorter than wavelength of visible rays, rather than an unevenness structure being optically large, without using a complicated mold.
However, an invention of JP-A-2011-70116 requires forming a mold having a fine unevenness structure of which a period is shorter than wavelength of visible rays to exhibit an excellent antireflection property. Further, it is necessary to continuously repeatedly use the mold while keeping the unevenness structure of the mold (that is, a complex process which is difficult in control is required), for continuously forming an antireflection film having a moth-eye structure.
The method recited in JP-B-5,038,748 method for forming an unevenness structure. However, an average distance between domains of a phase separation structure is 1-70 μm. Accordingly, it cannot be obtained an antireflection film having a moth-eye structure which requires a fine period of unevenness shorter than wavelength of visible rays, even if large unevenness in optical for antiglare property can be obtained.
In an invention of JP-A-2013-693, a period of unevenness of an obtained film is 1 μm, which exceeds wavelength of visible rays. Therefore, a reflection index at a shorter wavelength side in a wavelength range of visible rays is not lowered, and it cannot be said that the obtained film has a moth-eye structure. An invention of JP-A-2013-693 also requires a far infrared ray dryer for radiating far infrared ray, and therefore it is not simple manufacturing method.
To wrap up, it has been desired a technique for simply manufacturing an antireflection film having an unevenness structure (a moth-eye structure) of which a period is shorter than wavelength at a shorter wavelength side in a wavelength range of visible rays without using a complicated process such as using a mold or a special apparatus such as a far infrared ray dryer.
That is, an objective of the present invention is to provide an antireflection index layer having a moth-eye structure, which can be simply manufactured and has a low reflection index.
The present inventors have intensively studied the above problems. As a result, they have found that the following methods can form a moth-eye structure in which a sea part is a convex portion and an island part is a concave portion and have completed the present invention. The following method includes using (A) an ionizing radiation curable monomer including two or more polymerizable group in a molecular and (B) an ionizing radiation curable polymer including two or more polymerizable group, and having a SP value lower by 0.5 or more than the ionizing radiation curable monomer (A), viscosity larger by 100 mPa·s than the ionizing radiation curable monomer and a mass average molecular weight of 1000 or more so as to: adjust an area ratio of the sea part and the island part (area of the sea part/area of the island part) at outermost surface of an antireflection film to be 1.0 or more wherein the sea part is the curable monomer (A) and the island part is the curable polymer (B); form a size of the island part smaller than a short wavelength in a wavelength range of visible rays to obtain a phase separation structure in which a distance between island parts (i.e., a period of convexes) is smaller than a short wavelength in the wavelength range of visible rays; permeate the curable monomer (A) into an underlayer; and adjust the island part and the sea part to be a convex portion and a concave portion, respectively, by utilizing a characteristic that the curable polymer (B) is low shrinkage and the curable monomer (A) is high shrinkage at curing by using irradiation of ionizing radiation.